Ion thrusters or engines have played vital role in space propulsion for wide ranges of applications, such as low thrust precision attitude control, orbit transfer and interplanetary flights. One of crucial parameters of ion thrusters, which determine its applicability to specific missions, is the thrust density, the ratio of the thrust to the area of the exit nozzle/electrode. The high thrust density correlates with a smaller accelerator grid area that is essential in minimizing the construction, operation and lifting costs of the ion thrusters. Although the power to thrust conversion efficiency and Isp of ion thrusters can be much higher than conventional chemical thrusters, the ion thrusters currently are not used for missions requiring large thrusts in the order of multi megawatts, because their construction and lifting costs are prohibitive. Therefore, the methods of increasing thrust density of ion thrusters can significantly broaden their application scopes have been extensively sought for.
The operation of the ion thruster relies on acceleration of ions. Thus, the space-charge limitation of the ion acceleration process limits the thrust density. Currently, most of ion thrusters use atomic species, such as Xe or Hg making the ion thruster practical for only a limited range of missions. Extensive research efforts have attempted to increase thrust density to levels that would lead to an attractive ion thruster with wider applicability with the use of heavier ion species than Xe or Hg without success. The method of increasing thruster density can be guided by a physical theory by Child-Longmuir law, and according to this law, the thrust density, Ta, of an ion thruster can be given by:Ta∝mi2Isp4,  (1)where Isp is the specific impulse and mi is the ion mass of the propellant. For a specific mission with a fixed Isp, the higher the ion mass is, the higher is the thruster density. Because the thrust density is proportional to square of the ion mass, even small change in ion mass can increase the thrust density significantly. For example, the atomic mass of the most popular propellant Xe is 131, and any fuels with atomic or molecular mass greater than 131 would increase the thrust density over the current limit.
Molecular or cluster ions can potentially increase ion mass significantly, however, with highly increased probability of fragmentation, which negates the effect of increased ion mass on thrust density. Therefore, the usage of molecular or cluster ions for ion thrusters has not been successful until now. Fullerene clusters, such as C60, have much larger masses than Xe, yet under favorable thermodynamic conditions, they behave like atoms in terms of resisting fragmentation. In addition to their larger mass than that atomic species, fullerene clusters have lower ionization potentials, thus require lesser energy for ionization than atomic species. Fullerene clusters can be sublimated at relatively low temperatures without fragmentation, and their vapors behave like atomic vapors. Therefore, the usage of C60 for propellant for ion thrusters has been extensively investigated by researchers over two decades.
For example, C60 clusters, cardinal clusters among fullerenes, have a mass of 720. If thrust operation conditions are kept the same, the thrust density of C60 ions would be greater than Xe ions by a factor of (720/131)2˜30 according to Eq. 1. For example, a high thrust mission with a thruster beam power of 10 MW and Isp-5,000 with Xe as propellant would need a grid area of 18 m2, which is too large for economically viable construction and lift into space. If a similar ion thruster can be operated with C60 fuel, the required grid area decreases to 0.60 m2, which is economically viable for a wide range of space missions. The heavier fullerenes, such as C72 or C84 would have better size-reduction effects. The chemistry of fullerenes has recently produced extensive classes of fullerene derivatives, fullerene nanotubes, and fullerene nanotube derivatives. Successful usages of these large stable clusters will further increase the thruster density. Therefore, fullerene-family fuels may open new doors for electrostatic propulsion, if they can be successfully used in ion thrusters.
Extensive research and development efforts for fullerene ion thrusters have at best produced engines with undesirably low fuel usage efficiency due to serious propellant deposition and other problems resulting from premature fragmentation before full electrostatic acceleration. Previous state-of-the-art fullerene ion thrusters have used traditional ionization methods including DC and RF discharge plasmas. An example C60-based ion thruster system is described in U.S. Pat. No. 5,239,820, entitled “Electric Propulsion Using C60 Molecules,” issued Aug. 31, 1993, to Leifer et al., the prior usage of such ion thruster structures and operation methods with cathodes and hot filaments in DC or RF discharge chambers has not successful in realizing efficient and practical fullerene ion thrusters. A number of publications similar to the above mentioned C60 ion thruster system reported a failure of obtaining sufficiently high efficiency of fullerenes for rendering C60 ion thruster practically and economically viable. Other methods include the usage of charge exchange of fullerene with rare gas ions generated in discharge chambers in a modified configuration of Hall thrusters. An example such ion thruster system is described in Hruby, et al., “A High Thrust Density, C60 Cluster Ion, Thruster,” AFOSR Final Report No. 49620-94-C-0006, September 1996. Such approach also resulted in similar inefficiency of fullerene usage to the above mentioned references.
None of the existing approaches so far resulted in a practical fullerene ion thruster, mainly because their ionization methods for generating fullerene cluster ions induce extensive fragmentation of fullerene clusters resulting in very low efficiency fullerene usage. Therefore, other innovative ion thruster structures and operation methods have been sought for. The present invention solves these problems in existing fullerene ion thrusters with the use of VUV photoionization followed by thermal effusion of fullerene clusters, which has negligible fragmentation during ionization process, thus promises cost effective and practical fullerene ion thrusters for a wide range of space propulsion applications.